This invention relates to a semiconductor converter, provided with an improved cooling structure, such as an inverter or a rectifier provided with a heat generating element such as a resistor or a semiconductor element such as a thyrister or a gate turn-off thyrister (GTO).
Recently, in accordance with rapid development of power semiconductor elements, integrated circuits (IC), and large scale integration (LSI) technique, electronic parts or elements have been made more compact with high density. Moreover, the improvement in packing technique for assembling and wiring of these electronic parts or elements to operate as a combined unit or equipment makes compact or multi-functional a whole converter device or unit. In addition, the power semiconductor element of large capacity has been extremely developed, and accordingly, a power semiconductor element with a rated value of several thousands of amperes has been generally used. According to the technological development described above, there has been produced a semiconductor converter in which each one cubicle outputs several megavolt-amperes (MVA). Thus, the packing density of the electronic parts and the like in the semiconductor converter and the heat generated from the heat generating elements such as semiconductor elements or resistors have been increased in the form of an exponential function.
Accordingly, it becomes a significant problem or subject to improve a cooling method of the semiconductor converter to attain reliability of structure thereof.
FIG. 5 shows a diagram of a conventional circuit of a semiconductor converter assembled in an inverter circuit which converts a direct current (DC) into an alternating current (AC). As cooling means for such semiconductor converter, a cooling structure or unit of the type shown in FIGS. 6, 7 or 8 is generally used. The semiconductor converter shown in FIG. 5 essentially comprises a DC input reactor 1, a capacitor 2, and a plurality of semiconductor units 3 each consisting of a GTO, a diode and a snubber circuit with resistors 4 for the capacitor 2, which are electrically operatively connected.
FIG. 6 is a side view of a semiconductor converter of conventional type in which two sets of the circuits shown in FIG. 5 are installed. FIG. 7 is a cross sectional plan view taken along the line VII--VII shown in FIG. 6. In the semiconductor converter shown in FIG. 6, cooling is effected by circulating air through the operation of a cooling fan 5 secured to the ceiling of the semiconductor converter 9. The air for cooling is introduced into the semiconductor converter 9 through an air intaking port 8 with a filter 7 formed in a lower portion of a door 6 of the converter 9, and diverges to cool the respective semiconductor units 3 and then the resistors 4 disposed behind the semiconductor units 3, thereafter being discharged externally through an air discharging port 10. The reactor 1 and the capacitor 2 are located further behind the resistors 4, and guide plates 11a and 11b are located at upper and lower portions of the resistors 4 for effectively cooling the resistors 4, but these guide plates 11a and 11b may be eliminated as shown in FIG. 8. The resistance used for the resistors 4 of the snubber circuit has a certain relationship existing between the cooling velocity and the maximum working electric power as shown in FIG. 9, and accordingly, the number of the resistors 4 to be used in the converter is determined by the degrees of the working electric power and the cooling velocity.
The cooling structure for the conventional semiconductor converter of the character described above reveals the following disadvantages.
In the semiconductor converter having the cooling structure of the type shown in FIG. 6, since the resistors 4 are cooled by the cooling air diverging before cooling the semiconductor units 3, the velocity of the cooling air relative to the resistors 4 is reduced. Also with the semiconductor converter having the cooling structure of the type shown in FIG. 8, the resistors 4 located at relatively upper portions can be cooled by the cooling air having a relatively high velocity, whereas the resistors 4 located in relatively low portions are cooled by the cooling air with a velocity further lower than the velocity of the cooling air for cooling the resistors equipped with the guide plates 11a and 11b shown in FIG. 6. Since the number of the resistors 4 installed in the converter is determined by the lowest cooling velocity, a larger number of the resistors 4 are provided in the converter shown in FIG. 8 than that of the converter shown in FIG. 6.
Moreover, due to the development of the semiconductor converter of large capacity, the working electric power for the resistors 4 increases and the number of resistors to be used increases, accordingly. This tendency, however, does not comply with the requirement of the development of a compact structure of the semiconductor converter, and accordingly, it is impossible to increase the number of the resistors to be installed in the semiconductor converter in proportion to the increment of the working power. This fact requires increase of the velocity of the cooling air. In the converter shown in FIG. 6, however, it is required for the cooling fan 5 to have an increased capacity in order to increase the air velocity for cooling the resistors 4, and hence, it is necessary to increase the overall size of the cooling fan 5, which results in increase of the cost for the installation of the whole structure of the converter. In addition, since the increase of the cooling capacity of the cooling fan 5 in turn results in increase of the head loss at the filter 7, the semiconductor units 3 and the like in proportion to the square of the air velocity, effective increase of the velocity of the cooling air for cooling the resistors 4 in proportion to the increase of the cooling capacity of the fan 5 cannot be expected. Moreover, in the conventional cooling structure, the resistors 4 are located at directly downstream-side of the semiconductor units 3 for effectively cooling the same, so that the cooling air with an increased temperature which has cooled the semiconductor units 3 and the resistors 4 is directed to the capacitor 2. This involves a significant problem because the capacitor widely used in these days has very low heat resisting property, and accordingly, it is not desired to direct the cooling air with an increased temperature to the capacitor.